Healing time for bone incorporation in a cervical fusion (ACDF) depends upon the type of graft used. There are generally three types of graft used; autograft (your own bone), allograft (cadaver) and PEEK cages (a plastic cage with some of your own bone shavings in the center of the cage).
There are two pieces of information to be concerned with- time to healing and complete healing. If the graft does not heal- this is called a pseudoarthrosis or non-union. The chance of the non-union is higher with PEEK cages and allograft and diminished with autograft.
The time to healing for your own bone is about 6-8 weeks. The time to healing with allograft is about 3-4 months and with PEEK cages about 4 months.
The way to determine healing is with X-rays including flexion and extension films. If there is any uncertainty, then a CT scan can be performed which is much more accurate.
If the fusion does not initially heal, this is called a delayed union. There are various adjuncts to try to increase healing such as a bone stimulator. If the grafts will not incorporate (fuse), this is called a non-union.
Non-unions can be non-painful (stable) or painful. If painful, then redo surgery is required. Most times, this would be a posterior fusion of the non-united level but there are times a repeat anterior surgery would be required.